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Research and collaboration

Data analysis

A dense and hot droplet of strongly interacting matter is created in ultra-relativistic collision between two heavy nucleus (URHIC). The droplet then expands and cools, and finally decouples to final state particles that are seen by the detector. Experimental data in then analyzed from many different points of view and our group is specialized to two direction: study collectivity trough correlations of flow harmonics and correlations triggered by high momentum particles or jets.

Flow harmonics:

There are pressure differences between interior of the droplet and surrounding vacuum. Generated pressure gradients push matter into a collective motion. This results in, for example, an enhanced yield of particles into direction of stronger flow. These anisotropies are then studied via Fourier series analysis of the final hadron momentum distributions, as function of azimuthal angle with respect to event plane of the collision. In this situation, the Fourier coefficients are called as flow harmonics. We study different harmonics and correlations among them that have proven out to be a very rich source of information on the system.

Figure left [taken from Phys.Rev.Lett. 108 (2012) 252301] presents a simulated initial of the heavy ion collision. The initial state is anisotropic due to (i) geometrical overlap of the colliding nuclei and (ii) hot spots originating from fluctuations of the initial nucleon-nucleon configurations inside the nucleus and particle production in the primary interactions. Both cause subtle correlations between different flow harmonics that can be measured, see Figure right. Curves show model results with two different descriptions for the temperature dependence of the shear viscosity over temperature ratio in the QGP.

High-momentum correlations:

High momentum partons (quarks or gluons), generated in the primary interactions, do not thermalize. Instead they traverse trough the fireball losing energy along their way. This phenomena is called as jet quenching and it depends on transport properties of the QGP. In general, as partons cannot travel long distances in a free space, they will always hadronize before reaching the detector. The very high momentum of the original parton collimates the hadrons originating from it as a spray of final state hadrons called as a jet. Jets can be studied via reconstruction algorithms or two-particle correlations. Former aims to reconstruct the original parton momentum using various computational algorithms. These studies are well established in electron-positron and proton-proton collisions, and in recent years the field has developed strongly also in URHIC. In two-particle correlations one picks up a high momentum trigger hadron, that is likely to belong into a jet, and then studies correlations among associated particles with different kinematical cuts.

Figure shows how the ratio of per trigger normalized yield of associated particles measured in lead-lead to proton-proton collisions behaves as a function of pseudo-rapidity difference of the trigger and associated partiles, i.e. geometrical distance along collision axis. The measured ratio degreases in some combinations of trigger and associated momenta indicating that the jet is getting narrower in lead-lead collisions in certain regions of phase space. This result was unexpected, since models mostly predicted broadening due the medium interactions. Possible explanations can come from details of the kinematical collimation or stronger absorption of quarks to the medium, but the final explanation is not yet reached.

ALICE upgrade

ALICE detector will undergo major upgrade during the long shutdown 2 (LS2) period in LHC during years 2019-2020. After the LS2, LHC starts to run in higher luminosity and all detectors and electronics have meet the new requirements. Our group participates into two major activities during LS2.

Fast Interaction Trigger (FIT):

Every experiment needs a "trigger decision", i.e. when a good or desired collisions has happened and signals from the detector should be recorded. In the currently version of ALICE, so called minimum bias (MB) trigger is based on V0 detector that consists of two parts (V0A and V0C) on both sides of the interaction point at forward and backward direction. MB trigger is fired when simultaneous hit is seen in both V0A and V0C. Another, somewhat smaller, forward detector in ALICE is T0 that gives precise timing of the interaction. Jyväskylä participated strongly to design and building of the T0 detector, and its current running maintenance and operation (see next section).

During LS2, three forward detectors, aforementioned V0 and T0, and Forward Multiplicity Detector (FMD), will be replaced by one new detector - Forward Interaction Trigger (FIT). FIT is a very large international collaboration and lead by Jyväskylä.

TPC readout upgrade:

The main tracking device of the ALICE is the largest Time Projection Chamber (TPC) in the whole world. A technical detail in ALICE TPC is related to gating grid that is needed to prevent the ion backflow to the TPC gas. Problem is that the gating grid limits TPC readout rate to 500 Hz while the new requirement, after the LS2, to 50 kHz. To meet this goal, the whole readout of the ALICE TPC is redesigned around GEM-foil technology. Finland has a significant knowhow on GEM-foil Quality Assurance (QA) techniques based on optical scanning in the detector laboratory at the Helsinki Institute of Physics (HIP). We have constructed a base line QA technique to Helsinki, and the same setup is later copied into Wigner Institute in Budapest.

Detector maintenance and operation

Fast timing detector, T0:Jyväskylä provide very significant contribution to design and building the fast and precise timing detector, T0, in ALICE. T0 was one of the first detectors installed into ALICE and it has worked very reliably over the whole measurement period so far. Due to aging of the V0 detector, T0 has gained significance while it has served as a luminometer of ALICE and provided information on collision vertex position and veto of interaction between two different bunch crossings in the beam. Precise timing information is also needed in the particle identification based on time of flight measurements (TOF). Timing resolution of the ALICE TOF detector alone is order of 100 ns but when T0 signal is combined to the information, we can reach order of 30 ns resolution. We have actively participated into maintenance and operation of the T0 during the LHC running.

Single photon trigger system:

Besides minimum bias, all detectors aim to enhance number of rare events in the data samples. These can be, for example, significant upward fluctuation in multiplicity, an event that is more likely to contain heavy quarks states or jets. One of the important rare event triggers is a singly photon trigger that requires that an event contained at least one very energetic hit to electromagnetic calorimeter, EMCal. This hit is most likely a high energy photon, but it can be also an electron or a positron, or in some rare cases some other particle. Our group has designed and implemented the single photon trigger algorithm that is used in the ALICE EMCal's and we also maintain the trigger region unit (TRU) electronics that does the job.

Shifts and running of the experiment:

When beams are available, experiments collect data 24/7 in three shifts. Work needs so called central shifters that take care of the data taking and monitor its quality, and then detector experts that react as fast as possible to any problem with the hardware. All participating institutes make their fair share of the total shift load. Jyväskylä makes actively both central shifts and detector expert shifts for the T0.